Conventional glass compositions used in connection with forming optical devices, such as lenses, are known. In the interest of providing smaller, lighter lenses, it has been desirable to increase the refractive index and decrease the dispersion of the glass. Although high refractive index glasses can be made with the inclusion of heavy metals such as lead, these are not desirable elements for environmental reasons. Typically speaking, alternatives to the heavy metals tend to also raise the glass transition temperature (Tg), which makes it necessary to use correspondingly high temperature processing conditions to press-form the lenses. Indeed, the thermophysical characteristics of such glasses tend not to be optimal for press molding applications.
Specifically, the molds that are used under the high temperature forming conditions tend to be susceptible to release problems, thermal degradation and breakage, which negatively affect lens production. For example, the diamond-like coating on the mold surfaces of silicon carbide molds can be degraded, which reduces overall lens productivity and shortens the useful life of the molding apparatus. Heretofore, however, more emphasis has traditionally been placed on the importance of providing the desired optical properties over the thermophysical properties.
In view of the above, it is an object of the present invention to provide a glass composition for use in press molding aspherical lenses, for example, that has desirable optical properties, such as a high refractive index, while at the same time also exhibiting suitable thermophysical properties (e.g. Tg) so as to allow for lower processing temperatures in order improve lens productivity and extend the useful life if the molding apparatus.